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Watt

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This article is about the unit of power. For other uses, seeWatt (disambiguation).
"MWT" redirects here. For the former Australian theatre company, seeMelbourne Workers Theatre.
"Joules per second" redirects here, it should not be confused withJoule-second
watt
Unit systemSI
Unit ofpower
SymbolW
Named afterJames Watt
Conversions
1 Win...... is equal to...
SI base units1kg⋅m2⋅s−3
CGS units107erg⋅s−1
English Engineering Units0.7375621 ft⋅lbf/s = 0.001341022 hp

Thewatt(symbol:W) is the unit ofpowerorradiant fluxin theInternational System of Units (SI),equal to 1joulepersecondor 1 kg⋅m2⋅s−3.[1][2][3]It is used toquantifythe rate ofenergy transfer.The watt is named in honor ofJames Watt(1736–1819), an 18th-centuryScottishinventor,mechanical engineer,andchemistwho improved theNewcomen enginewith his ownsteam enginein 1776. Watt's invention was fundamental for theIndustrial Revolution.

Overview[edit]

When an object'svelocityis held constant at onemeter per secondagainst a constant opposing force of onenewton,the rate at whichworkis done is one watt.

In terms ofelectromagnetism,one watt is the rate at whichelectrical workis performed when a current of oneampere(A) flows across an electricalpotential differenceof onevolt(V), meaning the watt is equivalent to thevolt-ampere(the latter unit, however, is used for a different quantity from thereal powerof an electrical circuit).

Two additionalunit conversionsfor watt can be found using the above equation andOhm's law. whereohm() is theSI derived unitofelectrical resistance.

Examples[edit]

  • A person having a mass of 100 kg who climbs a 3-meter-high ladder in 5 seconds is doing work at a rate of about 600 watts. Mass times acceleration due togravitytimes height divided by the time it takes to lift the object to the given height gives therate of doing workorpower.[i]
  • A laborer over the course of an eight-hour day can sustain an average output of about 75 watts; higher power levels can be achieved for short intervals and by athletes.[4]

Origin and adoption as an SI unit[edit]

The watt is named after the Scottish inventorJames Watt.[5]The unit name was proposed byC. William Siemensin August 1882 in his President's Address to the Fifty-Second Congress of theBritish Association for the Advancement of Science.[6]Noting that units in thepractical system of unitswere named after leading physicists, Siemens proposed thatwattmight be an appropriate name for a unit of power.[7]Siemens defined the unit within the existing system of practical units as "the power conveyed by a current of anAmpèrethrough the difference of potential of a Volt ".[8]

In October 1908, at the International Conference on Electric Units and Standards in London,[9]so-calledinternationaldefinitions were established for practical electrical units.[10]Siemens' definition was adopted as theinternationalwatt. (Also used:1 A2×1 Ω.)[5]The watt was defined as equal to 107units of power in thepractical systemof units.[10]The"international units"were dominant from 1909 until 1948. After the 9thGeneral Conference on Weights and Measuresin 1948, theinternationalwatt was redefined from practical units to absolute units (i.e., using only length, mass, and time). Concretely, this meant that 1 watt was defined as the quantity of energy transferred in a unit of time, namely 1 J/s. In this new definition, 1absolutewatt = 1.00019internationalwatts. Texts written before 1948 are likely to be using theinternationalwatt, which implies caution when comparing numerical values from this period with the post-1948 watt.[5]In 1960, the 11th General Conference on Weights and Measures adopted theabsolutewatt into theInternational System of Units(SI) as the unit of power.[11]

Multiples[edit]

For additional examples of magnitude for multiples and submultiples of the watt, seeOrders of magnitude (power).
SI multiples of watt (W)
Submultiples Multiples
Value SI symbol Name Value SI symbol Name
10−1W dW deciwatt 101W daW decawatt
10−2W cW centiwatt 102W hW hectowatt
10−3W mW milliwatt 103W kW kilowatt
10−6W μW microwatt 106W MW megawatt
10−9W nW nanowatt 109W GW gigawatt
10−12W pW picowatt 1012W TW terawatt
10−15W fW femtowatt 1015W PW petawatt
10−18W aW attowatt 1018W EW exawatt
10−21W zW zeptowatt 1021W ZW zettawatt
10−24W yW yoctowatt 1024W YW yottawatt
10−27W rW rontowatt 1027W RW ronnawatt
10−30W qW quectowatt 1030W QW quettawatt
Common multiples are inboldface
Attowatt
The sound intensity in water corresponding to the international standard referencesound pressureof 1μPais approximately 0.65 aW/m2.[12]
Femtowatt
Powers measured in femtowatts are typically found in references toradioandradarreceivers. For example, meaningfulFM tunerperformance figures for sensitivity, quieting andsignal-to-noiserequire that theRFenergy applied to the antenna input be specified. These input levels are often stated in dBf (decibelsreferenced to 1 femtowatt). This is 0.2739 microvolts across a 75-ohm load or 0.5477 microvolt across a 300-ohm load; the specification takes into account the RFinput impedanceof the tuner.
Picowatt
Powers measured in picowatts are typically used in reference to radio and radar receivers,acousticsand in the science ofradio astronomy.One picowatt is the international standard reference value ofsound powerwhen this quantity is expressed in decibels.[13]
Nanowatt
Powers measured in nanowatts are also typically used in reference to radio and radar receivers.
Microwatt
Powers measured in microwatts are typically stated inmedical instrumentationsystems such as theelectroencephalograph(EEG) and theelectrocardiograph(ECG), in a wide variety of scientific and engineering instruments and also in reference to radio and radar receivers. Compactsolar cellsfor devices such ascalculatorsandwatchesare typically measured in microwatts.[14]
Milliwatt
A typicallaser pointeroutputs about five milliwatts of light power, whereas a typicalhearing aiduses less than one milliwatt.[15]Audio signalsand other electronic signal levels are often measured indBm,referenced to one milliwatt.
Kilowatt
"Kilowatt" and "Kilowatts" redirect here. For the musician James Watts, seeKiloWatts (musician).
"kW" redirects here. For other uses, seekW (disambiguation).
The kilowatt is typically used to express the output power ofenginesand the power ofelectric motors,tools, machines, and heaters. It is also a common unit used to express theelectromagneticpower output of broadcast radio and televisiontransmitters.
One kilowatt is approximately equal to 1.34horsepower.A small electric heater with oneheating elementcan use 1 kilowatt. The averageelectric power consumptionof a household in the United States is about 1 kilowatt.[ii]
A surface area of 1 square meter on Earth receives typically about one kilowatt of sunlight from the Sun (thesolar irradiance) (on a clear day at midday, close to the equator).[17]
Megawatt
Many events or machines produce or sustain the conversion of energy on this scale, including large electric motors; large warships such as aircraft carriers, cruisers, and submarines; largeserver farmsordata centers;and some scientific research equipment, such assupercolliders,and the output pulses of very large lasers. A large residential or commercial building may use several megawatts in electric power and heat. On railways, modern high-poweredelectric locomotivestypically have a peak power output of5 or 6 MW,while some produce much more. TheEurostar e300,for example, uses more than12 MW,while heavydiesel-electric locomotivestypically produce and use3 and 5 MW.U.S.nuclear power plantshave net summer capacities between about500 and 1300 MW.[18]: 84–101 
The earliest citing of the megawatt in theOxford English Dictionary(OED) is a reference in the 1900Webster's International Dictionary of the English Language.TheOEDalso states thatmegawattappeared in a 28 November 1947 article in the journalScience(506:2).
AUnited States Department of Energyvideo explaining gigawatts
Gigawatt
A gigawatt is typical average power for an industrial city of one million habitants and also the output of a large power station. The GW unit is thus used for large power plants andpower grids.For example, by the end of 2010, power shortages in China's Shanxi province were expected to increase to 5–6 GW[19]and the installation capacity of wind power in Germany was 25.8 GW.[20]The largest unit (out of four) of the BelgianDoel Nuclear Power Stationhas a peak output of 1.04 GW.[21]HVDC convertershave been built with power ratings of up to 2 GW.[22]
Terawatt
Theprimary energyused by humans worldwide was about 160,000 terawatt-hours in 2019, corresponding to an average continuous power consumption of 18 TW that year.[23]Earthitself emits 47±2 TW,[24]far less than the energy received from solar radiation. The most powerful lasers from the mid-1960s to the mid-1990s produced power in terawatts, but only fornanosecondintervals. The average lightning strike peaks at 1 TW, but these strikes only last for 30microseconds.
Petawatt
A petawatt can be produced by the current generation of lasers for time scales on the order of picoseconds. One such laser isLawrence Livermore'sNova laser,which achieved a power output of 1.25 PW by a process calledchirped pulse amplification.The duration of the pulse was roughly 0.5ps,giving a total energy of 600 J.[25]Another example is the Laser for Fast Ignition Experiments (LFEX) at the Institute of Laser Engineering (ILE),Osaka University,which achieved a power output of 2 PW for a duration of approximately 1ps.[26][27]
Based on the average total solar irradiance of 1.361 kW/m2,[28]the total power of sunlight striking Earth's atmosphere is estimated at 174 PW. The planet's average rate of global warming, measured asEarth's energy imbalance,reached about 0.5 PW (0.3% of incident solar power) by 2019.[29]
Yottawatt
The power output of the Sun is 382.8 YW, about 2 billion times the power estimated to reach Earth's atmosphere.[30]

Conventions in the electric power industry[edit]

In theelectric power industry,megawatt electrical(MWe[31]or MWe)[32]refers by convention to theelectric powerproduced by a generator, whilemegawatt thermalorthermal megawatt[33](MWt, MWt,or MWth, MWth) refers tothermal powerproduced by the plant. For example, theEmbalse nuclear power plantin Argentina uses afission reactorto generate 2,109 MWt(i.e. heat), which creates steam to drive a turbine, which generates 648 MWe(i.e. electricity). OtherSI prefixesare sometimes used, for examplegigawatt electrical(GWe). TheInternational Bureau of Weights and Measures,which maintains the SI-standard, states that further information about a quantity should not be attached to the unit symbol but instead to the quantity symbol (e.g.,Pth= 270 Wrather thanP= 270 Wth) and so these unit symbols are non-SI.[34]In compliance with SI, the energy companyØrsted A/Suses the unit megawatt for produced electrical power and the equivalent unitmegajouleper second for delivered heating power in acombined heat and powerstation such asAvedøre Power Station.[35]

When describingalternating current(AC) electricity, another distinction is made between the watt and thevolt-ampere.While these units are equivalent for simpleresistivecircuits,they differ when loads exhibitelectrical reactance.

Radio transmission[edit]

Main article:Effective radiated power

Radio stationsusually report the power of theirtransmittersin units of watts, referring to theeffective radiated power.This refers to the power that ahalf-wavedipole antennawould need to radiate to match the intensity of the transmitter'smain lobe.

Distinction between watts and watt-hours[edit]

The termspowerandenergyare closely related but distinct physical quantities. Power is the rate at which energy is generated or consumed and hence is measured in units (e.g. watts) that represent energyper unit time.

For example, when alight bulbwith apower ratingof100Wis turned on for one hour, the energy used is 100watt hours(W·h), 0.1 kilowatt hour, or 360kJ.This same amount of energy would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours.

Power stationsare rated using units of power, typically megawatts or gigawatts (for example, theThree Gorges Damin China is rated at approximately 22 gigawatts). This reflects the maximum power output it can achieve at any point in time. A power station's annual energy output, however, would be recorded using units of energy (not power), typically gigawatt hours. Major energy production or consumption is often expressed asterawatt hoursfor a given period; often a calendar year or financial year. One terawatt hour of energy is equal to a sustained power delivery of one terawatt for one hour, or approximately 114 megawatts for a period of one year:

Power output = energy / time
1 terawatt hour per year =1×1012W·h/ (365 days × 24 hours per day) ≈ 114 million watts,

equivalent to approximately 114 megawatts of constant power output.

Thewatt-secondis a unit of energy, equal to thejoule.One kilowatt hour is 3,600,000 watt seconds.

While a watt per hour is a unit of rate of change of power with time,[iii]it is not correct to refer to a watt (or watt-hour) as a watt per hour.[36]

See also[edit]

Explanatory notes[edit]

  1. ^The energy in climbing the stairs is given bymgh.Settingm= 100 kg,g= 9.8 m/s2andh= 3 mgives 2940 J. Dividing this by the time taken (5 s) gives a power of 588 W.
  2. ^Average household electric power consumption is 1.19 kW in the US, 0.53 kW in the UK. In India it is 0.13 kW (urban) and 0.03 kW (rural) – computed from GJ figures quoted by Nakagami, Murakoshi and Iwafune.[16]
  3. ^Watts per hour refers to therate of changeof power being used (or generated). For example, a power plant that changes its power output from 100 MW to 200 MW in 15 minutes would have a ramp-up rate of 400 MW/h. Gigawatts per hour are used to characterize the ramp-up required of thepower plantson an electric grid to compensate for loss of output from other sources, such as whensolar powergeneration drops to zero as the sun sets. Seeduck curve.

References[edit]

  1. ^Newell, David B; Tiesinga, Eite (2019).The international system of units (SI)(PDF)(Report). Gaithersburg, MD: National Institute of Standards and Technology.doi:10.6028/nist.sp.330-2019.§2.3.4, Table 4.
  2. ^Yildiz, I.; Liu, Y. (2018). "Energy units, conversions, and dimensional analysis". In Dincer, I. (ed.).Comprehensive energy systems. Vol 1: Energy fundamentals.Elsevier. pp. 12–13.ISBN9780128149256.
  3. ^International Bureau of Weights and Measures(2006),The International System of Units (SI)(PDF)(8th ed.), pp. 118, 144,ISBN92-822-2213-6,archived(PDF)from the original on 2021-06-04,retrieved2021-12-16
  4. ^Avallone, Eugene A; et al., eds. (2007),Marks' Standard Handbook for Mechanical Engineers(11th ed.), New York: Mc-Graw Hill, pp. 9–4,ISBN978-0-07-142867-5.
  5. ^abcKlein, Herbert Arthur (1988) [1974].The Science of measurement: A historical survey.New York: Dover. p. 239.ISBN9780486144979.
  6. ^"Address by C. William Siemens".Report of the Fifty-Second meeting of the British Association for the Advancement of Science.Vol. 52. London: John Murray. 1883. pp. 1–33.
  7. ^Siemens supported his proposal by asserting that Watt was the first who "had a clear physical conception of power, and gave a rational method for measuring it"."Siemens, 1883, p. 6"
  8. ^Report of the British Association for the Advancement of Science.Vol. 52nd Meeting (1882). April 3, 1883.
  9. ^Tunbridge, P. (1992).Lord Kelvin: His Influence on Electrical Measurements and Units.Peter Peregrinus: London. p. 51.ISBN0-86341-237-8.
  10. ^abFleming, John Ambrose(1911)."Units, Physical".InChisholm, Hugh(ed.).Encyclopædia Britannica.Vol. 27 (11th ed.). Cambridge University Press. pp. 738–745, see page 742.
  11. ^"Resolution 12 of the 11th CGPM (1960)".Bureau International des Poids et Mesures (BIPM). Archived fromthe originalon April 20, 2020.Retrieved9 April2018.
  12. ^Ainslie, M. A. (2015). A century of sonar: Planetary oceanography, underwater noise monitoring, and the terminology of underwater sound. Acoustics Today.
  13. ^Morfey, C.L. (2001). Dictionary of Acoustics.
  14. ^"Bye-Bye Batteries: Radio Waves as a Low-Power Source",The New York Times,Jul 18, 2010,archivedfrom the original on 2017-03-21.
  15. ^Stetzler, Trudy; Magotra, Neeraj; Gelabert, Pedro; Kasthuri, Preethi; Bangalore, Sridevi."Low-Power Real-Time Programmable DSP Development Platform for Digital Hearing Aids".Datasheet Archive.Archivedfrom the original on 3 March 2011.Retrieved8 February2010.
  16. ^Nakagami, Hidetoshi; Murakoshi, Chiharu; Iwafune, Yumiko (2008).International Comparison of Household Energy Consumption and Its Indicator(PDF).ACEEE Summer Study on Energy Efficiency in Buildings.Pacific Grove, California:American Council for an Energy-Efficient Economy. Figure 3. Energy Consumption per Household by Fuel Type. 8:214–8:224.Archived(PDF)from the original on 9 January 2015.Retrieved14 February2013.
  17. ^Elena Papadopoulou,Photovoltaic Industrial Systems: An Environmental Approach,Springer 2011ISBN3642163017,p.153
  18. ^"Appendix A | U.S. Commercial Nuclear Power Reactors"(PDF).2007–2008 Information Digest (Report). Vol. 19.United States Nuclear Regulatory Commission.1 August 2007. pp. 84–101. Archived fromthe original(PDF)on 16 February 2008.Retrieved27 December2021.
  19. ^Bai, Jim; Chen, Aizhu (11 November 2010). Lewis, Chris (ed.)."China's Shanxi to face 5–6 GW power shortage by yr-end – paper".Peking: Reuters.
  20. ^"Not on my beach, please".The Economist.19 August 2010.Archivedfrom the original on 24 August 2010.
  21. ^"Chiffres clés"[Key numbers].Electrabel.Who are we: Nuclear (in French). 2011. Archived fromthe originalon 2011-07-10.
  22. ^Davidson, CC; Preedy, RM; Cao, J; Zhou, C; Fu, J (October 2010), "Ultra-High-Power Thyristor Valves for HVDC in Developing Countries",9th International Conference on AC/DC Power Transmission,London:IET.
  23. ^Hannah Ritchie;Max Roser(2020)."Global Direct Primary Energy Consumption".Our World in Data.Published online at OurWorldInData.org.Retrieved2020-02-09.
  24. ^Davies, J. H.; Davies, D. R. (2010-02-22)."Earth's surface heat flux".Solid Earth.1(1): 5–24.Bibcode:2010SolE....1....5D.doi:10.5194/se-1-5-2010.ISSN1869-9510.
  25. ^"Crossing the Petawatt threshold".Livermore,CA:Lawrence Livermore National Laboratory.Archivedfrom the original on 15 September 2012.Retrieved19 June2012.
  26. ^World's most powerful laser: 2 000 trillion watts. What's it?,IFL Science, 12 August 2015,archivedfrom the original on 2015-08-22.
  27. ^Eureka alert(publicity release), Aug 2015,archivedfrom the original on 2015-08-08.
  28. ^"Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present".CH:PMODWRC.Archivedfrom the original on 2011-08-30.Retrieved2005-10-05.
  29. ^Loeb, Norman G.; Johnson, Gregory C.; Thorsen, Tyler J.; Lyman, John M.; et al. (15 June 2021)."Satellite and Ocean Data Reveal Marked Increase in Earth's Heating Rate".Geophysical Research Letters.48(13).Bibcode:2021GeoRL..4893047L.doi:10.1029/2021GL093047.
  30. ^Williams, David R."Sun Fact Sheet".nasa.gov.NASA.Retrieved26 February2022.
  31. ^Rowlett, Russ."How Many? A Dictionary of Units of Measurement. M".University of North Carolina at Chapel Hill.Archivedfrom the original on 2011-09-04.Retrieved2017-03-04.
  32. ^ Cleveland, CJ (2007)."Watt".Encyclopedia of Earth.
  33. ^ "Solar Energy Grew at a Record Pace in 2008 (excerpt from EERE Network News".US:Department of Energy). 25 March 2009.Archivedfrom the original on 18 October 2011.
  34. ^International Bureau of Weights and Measures(2006),The International System of Units (SI)(PDF)(8th ed.), p. 132,ISBN92-822-2213-6,archived(PDF)from the original on 2021-06-04,retrieved2021-12-16
  35. ^"Avedøre Power Station (Avedøre værket) ".DONG Energy.Archived fromthe originalon 2014-03-17.Retrieved2014-03-17.
  36. ^"Inverter Selection".Northern Arizona Wind and Sun.Archivedfrom the original on 1 May 2009.Retrieved27 March2009.

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